Sinus P waves such as P1, P6, P7 and P19-P21 are 0.16 seconds in duration with an IACD manifesting two negative deflections in V1 separated by 0.08 seconds and a terminal negative deflection in Leads 2, 3, and aVF. The latter is due to retrograde activation of the LA (RALA)(1,2,3) and was something that could not have occurred unless there was either a total block(2) or a significantly greater degree of conduction delay in the path to the high left atrium(4), (BB), than there was in the path to the low LA, the part of the LA normally depolarized last(4). P11 and P22, which share the same contour in V1 and which occur at the expected time of the next sinus P wave are probably sinus P waves. These have almost the same contour in V1 for the first 0.08 seconds as sinus P waves that have RALA (because this represents mainly RA depolarization in P11 and P22). P11 and P22 are, however, shorter measuring 0.10 seconds because of the absence of RALA resulting from capture of the high LA via BB. Consistent with this idea is the small upright deflection in Lead I which occurs 0.08 seconds later relative to its RA predecessor in P19-P21 than the small upright deflection in Lead 1 relative to its RA predecessor in P-22. The later upright deflection is due to R-L depolarization of the left atrium which can be seen from the fact that it occurs. 06 seconds after the beginning of the P22 in Lead 2 and also from the fact that there is an increase in amplitude of the Pwaves in Leads 2 and 3 in P22 compared with the Pwaves with RALA in these same leads because it indicates high to low left atrial depolarization as a result of conduction through BB.

Since the second atrium in the atrial conduction sequence starts to be depolarized at the interatrial septum the direction of conduction through this atrial chamber can therefore be used to determine on which side of the interatrial septum the second atrium lies. Therefore, the small upright deflection seen in Lead I in P19-P21 coincident with the nadir of the RALA indicates right-to-left depolarization of the LA during RALA, confirming situs solitus.

P4 and P12 have in addition to the dome dart configuration, which is considered definitive for a LA origin(6,7), an IACD with P4 being about 0.14 seconds. Confirmation that P2, P3, and P4 are LA P wave comes from the negative deflections in Lead 1 coincident with the peaks of the dart part of those P waves which indicates a left-to-right depolarization of the RA, because in this case which is without a significant intra-atrial conduction delay(3) as can be seen from the normal duration of P22 but which is situs solitus left-to-right conduction through the entire second atrium in the atrial depolarization sequence identifies the latter chamber as the RA and the rhythm as left atrial. LA P waves may be difficult to see in surface leads(8) and this may account for the lack of a discernable LA P wave in P3.

P9 is most probably a right atrial fusion beat because P9 not only occurs at the expected time of the next Sinus Pwave but is followed by a P9-R9 interval which is too short to say that R9 was conducted from a SAN P9 because the conducted SAN Pwaves P5,P6,P7,P8 have a much longer PR interval while the P9-R9 interval is much the same length as the dart-R intervals seen in the LA beats P2,P3,P4. Therefore, P9 probably represents a right atrial fusion beat between a SAN P wave and a LA Pwave especially since no other eptopic activity from another origin was seen in the cardiogram.

The P13-P18 interval is four times the P13-P14 interval. The transseptal time from premature LA P15 allows the Sino Atiral Node (SAN) junction to be depolarized in its relative rather than its absolute refractory period causing retrograde concealed conduction into the SAN junction. This in turn causes a first degree antigrade conduction delay(9) in the SAN16-P16 interval which perpetuates as an anitgrade conduction delay in the SAN17-P17 interval. Since the P11-P13 interval is almost exactly two times the P10-P11 interval retrograde sinus node depolarization with temporary SAN depression following LAP12 is unlikely while retrograde concealed conduction into the SAN junction as that following LAP15 or RA refractoriness causing an antegrade block of the SAN-12 discharge is the most likely possibility including that of SAN junction interference.

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